This invention relates to a fuel gas steam reformer assemblage for reforming heavier hydrocarbon fuels such as methane, methanol or ethanol, but especially heavier fuels such as propane, butane, gasoline and diesel fuel, and converting them to a hydrogen-rich fuel stream suitable for use in a fuel cell power plant. More particularly, this invention relates to an autotheormal fuel gas steam reformer assemblage which employs a pre-catalyst bed mixing apparatus that provides an essentially uniform fuel/steam/air mixture, wherein at least 90% of the fuel, steam, and air are thoroughly admixed, for introduction into the catalyst bed in the reformer. The steam reformer assemblage of this invention is suitable for use in mobile applications.
Fuel cell power plants include fuel gas steam reformers which are operable to catalytically convert a fuel gas, such as natural gas or heavier hydrocarbons, into the primary constituents of hydrogen and carbon dioxide. The conversion involves passing a mixture of the fuel gas and steam, and, in, certain applications air/oxygen and steam, through a catalytic bed which is heated to a reforming temperature that varies, depending upon the fuel being reformed. A catalyst typically used is a nickel catalyst which is deposited on alumina pellets. Of the three types of reformers most commonly used for providing a hydrogenrich gas stream to fuel cell power plants, tubular thermal steam reformers, autothermal reformers, and catalyzed wall reformers, the autothermal reformer has a need for rapid mixing capabilities in order to thoroughly mix the fuel-steam and air prior to entrance into the reformer catalyst bed. U.S. Pat. No. 4,451,578, granted May 29, 1984 contains a discussion of autothermal reforming assemblages, and is incorporated-herein in its entirety. The autothermal reformer assembly described in the ""578 patent utilizes catalyzed alumina pellets. In the design of auto-thermal reformers for hydrogen-fueled fuel cell systems, thereis a need for rapid and thorough mixing of the reactants (air,. steam and fuel) prior to entry of the reactants into the catalyst bed. The autotherrmal reformers require a mixture of steam, fuel and air in order to operate properly. These reformers are desirable for use in mobile applications, such as in vehicles which are powered by electricity generated by a fuel cell power plant. The reason for this is that autothermal reformers can be compact, simple in design, and are better suited for operation with a fuel such as gasoline or diesel fuel. One requirement for a fuel processing system that is suitable for use in mobile applications is that the system should be as compact as possible, thus, the mixing of the steam, fuel and air constituents should be accomplished in as as compact an envelope as possible. Once the constituents are mixed, the residence time in the mixer must be limited to prevent carbon deposition. The problem encountered with such a compact assemblage is how to achieve the thorough degree of mixing needed in the autothermal reformer in as short a time and distance as possible with oxygen to carbon ratios as low as 0.30 to 0.35 for hydrocarbon fuels, and as low as 0.14 to 0.20 for methanol and ethanol fuels.
This invention relates to a compact autothermal reformer assemblage which is operable to reform relatively heavy hydrocarbon fuels such as propane, butane, gasoline, diesel. fuel, JP-4, JP-5 and JP-8, for example. In the reformer assemblage of this invention, air and steam are mixed in a premixing section prior to entering the auto-thermal reformer section of the assemblage. The reformer section includes a fuel, steam and air mixing station and the reforming catalyst bed. The catalyst bed care be a two stage bed, the first stage being, for example, an iron oxide catalyst stage, and the second stage being, for example, a nickel catalyst stage. The second stage could contain other catalysts, such as noble metal catalysts including rhodium, platinum, palladium, or a mixture of these catalysts. Alternatively, the catalyst bed could be a single stage bed with a noble metal catalyst, preferably rhodium, or a mixed rhodium/platinum catalyst.
The fuel is fed into the mixing station either alone or with some steam for improved vaporization, and an air/steam mixture (the oxidizer) is introduced into the mixing station. To minimize the mixing section length, the fuel-steam-air mixture must form rapidly and thoroughly. Minimizing mixing section length reduces the size of the assemblage, which is an important consideration for automotive applications. Minimizing the mixing section also reduces the residence time of the fuel, steam and air mixture in the mixer, thereby minimizing the risk of auto-ignition and of carbon formation prior to the mixture""s entering the catalyst bed.
The autothermal reformer assemblage of this invention is preferably, but not necessarily cylindrical or oval in shape and, as noted above, is associated with an air-steam premixing station. A stream of a relatively heavy hydrocarbon fuel, such as gasoline, diesel fuel, or JP fuel, with or without steam, is fed into one or more mixing tubes which form a part of the mixer station. The mixing tubes pass through a manifold which receives a steam/oxidant mixture. The oxidant is typically air. The fuel stream passes axially through the mixing tube(s), and the air/steam mixture enters the tube(s) from the air/steam manifold through sets of tangential openings formed in the tube(s). The mixing tube openings will include at least two sets of openings one of which will induce a clockwise air/steam/fuel swirl in the mixture flowing through the mixing tubes, and the other of which will induce a counter clockwise air/steam/fuel swirl in the mixture flowing through the mixing tubes. The counteracting mixture swirls result in a thorough mixing of the air/steam and fuel components which can be achieved relatively quickly and in a relatively compact envelope. The air/steam stream swirls the fuel stream first in one direction and then in the opposite direction, thus resulting in an essentially homogeneous fuel-steam-air mixture by the time that the mixture exits from the mixing tubes and enters into the catalyst bed.
The use of tangential mixing tube openings causes an initial intense mixing by creating a very high shear between the reactant streams and forces the heavier fuel stream to mix by accelerating it through the lighter air/steam stream. A counter swirl results in increased mixing by creating instabilities in the flow stream without the inclusion of baffles in the mixing tubes, which would tend to become fouled with carbonaceous deposits from the fuel. The high speed mixing action achieved in the assembly also eliminates fuel fouling of the mixing tube walls by minimizing fuel contact with the mixing tube walls.
Rapid mixing is required to prevent carbon formation because soon after air is added to the fuel stream, the gas mixture temperature starts to rise and the fuel wants to crack to form carbon. This carbon formation eventually plugs the reactor and prevents flow through the reactor. It is necessary to mix rapidly and get the fully mixed fuel/steam/air mixture to the catalyst to reform catalytically to hydrogen rather to react homogeneously and form carbon. As the mixer residence time increases, the amount of oxygen required to prevent carbon in the autothermal reformer increases significantly,
In order to prevent carbon formation in the catalyst bed itself, the gases must be fully mixed before contacting the catalyst, and one way to accomplish this result is by the generation of a swirling gas flow in the mixing tubes. The inclusion of an oppositely swirling enhances mixing as well as negating the swirl, thus providing a relatively uniform gas flow stream entering the catalyst bed. This minimizes disruption of the catalyst bed particles by the gas mixture as the latter flows into the catalyst bed. In order to produce maximum mixing of the two gas streams, it is desirable to design a mixer that will produce an air/steam,gas stream velocity which is at least three times the velocity of the fuel gas stream flowing through the mixing tubes. The degree of circumferential penetration of the tangential jets into the axial fuel flow stream can be controlled by the ratio of the kinetic energies of the tangentially flowing jet streams to the axially flowing fuel gas stream. If a single row of jets is used, then the penetration distance can be such that the jet swirls around the mixer tube several times; however, if two counter jets are used as described herein, then the first set of jets should be designed such that the swirling penetration from the first set of jets does not continue past the second set of jets.
It is therefore an object of this invention to provide an air/steam/fuel mixing method and system which achieves satisfactory mixing of two different composition streams in a relatively short period of time so as to minimize carbon deposition in the mixing system.
It is a further object of this invention to provide a method and system of the character described wherein one of the streams is swirled into the other of the streams in mixing tubes.
It is yet another object of this invention to provide a method and system of the character described wherein the air/steam/fuel mixture leaves the mixing tubes with minimal or no internal swirl.
These and other objects and advantages of the invention will be more readily understood from the following detailed description of a specific embodiment of the invention when taken in conjunction with the accompanying drawings, in which: